The Environmental Protection Agency, moving quickly to keep its new acting administrator from starting his tenure behind bars, yesterday issued tough new regulations to curb airborne radiation from nuclear weapons facilities, power plants and phosphorus mines.

The agency had scrapped the regulations last October, contending that the health risks from airborne radioactive particles were "relatively trivial" because so few people live around the sites.

That action drew a contempt order Dec. 11 from U.S. District Court Judge William H. Orrick in San Francisco, who four years earlier had ordered the agency to regulate industrial radiation. Yesterday, Supreme Court Justice William H. Rehnquist denied requests from the Reagan administration and a phosphorous mining group to suspend Orrick's ruling.

As a result, the agency was forced to issue the regulations immediately, essentially to prevent acting administrator Lee M. Thomas from going to jail. While former administrator William D. Ruckelshaus had been found in contempt of court, the order automatically transferred to Thomas last week when he became acting head of the agency.

"We're only doing this to purge ourselves of the contempt order," said Joseph A. Cannon, head of the agency's air and radiation division.

The EPA said last year that current emissions of radioactive particles, called radionuclides, carried a one-in-1,000 risk of cancer for persons exposed for a lifetime. The agency said that translated into fewer than one cancer death every 13 years, because most of the sites are in sparsely populated western states.

The decision was controversial because it was taken under a section of the Clean Air Act that requires the EPA to regulate hazardous air pollutants stringently enough to protect the public "with an ample margin of safety."

EPA officials have argued that they cannot comply with that provision without setting impossibly stringent rules that could force some industries out of business. Instead, the agency has attempted to set standards according to what it believes is a reasonably low risk.

"We think the thing we did in the first instance was the right thing," Cannon said. "We thought it was correct and protective of public health."